SYNOPSIS

DESCRIPTION

BIO_f_md() returns the message digest BIO method. This is a filter
BIO that digests any data passed through it, it is a BIO wrapper
for the digest routines EVP_DigestInit(), EVP_DigestUpdate()
and EVP_DigestFinal().

Any data written or read through a digest BIO using BIO_read() and
BIO_write() is digested.

BIO_gets(), if its size parameter is large enough finishes the
digest calculation and returns the digest value. BIO_puts() is
not supported.

BIO_reset() reinitialises a digest BIO.

BIO_set_md() sets the message digest of BIOb to md: this
must be called to initialize a digest BIO before any data is
passed through it. It is a BIO_ctrl() macro.

BIO_get_md() places the a pointer to the digest BIOs digest method
in mdp, it is a BIO_ctrl() macro.

BIO_get_md_ctx() returns the digest BIOs context into mdcp.

NOTES

The context returned by BIO_get_md_ctx() can be used in calls
to EVP_DigestFinal() and also the signature routines EVP_SignFinal()
and EVP_VerifyFinal().

The context returned by BIO_get_md_ctx() is an internal context
structure. Changes made to this context will affect the digest
BIO itself and the context pointer will become invalid when the digest
BIO is freed.

After the digest has been retrieved from a digest BIO it must be
reinitialized by calling BIO_reset(), or BIO_set_md() before any more
data is passed through it.

If an application needs to call BIO_gets() or BIO_puts() through
a chain containing digest BIOs then this can be done by prepending
a buffering BIO.

RETURN VALUES

BIO_f_md() returns the digest BIO method.

BIO_set_md(), BIO_get_md() and BIO_md_ctx() return 1 for success and
0 for failure.

EXAMPLES

The following example creates a BIO chain containing an SHA1 and MD5
digest BIO and passes the string ``Hello World'' through it. Error
checking has been omitted for clarity.

BUGS

The lack of support for BIO_puts() and the non standard behaviour of
BIO_gets() could be regarded as anomalous. It could be argued that BIO_gets()
and BIO_puts() should be passed to the next BIO in the chain and digest
the data passed through and that digests should be retrieved using a
separate BIO_ctrl() call.